1. Field of the Invention
The present invention relates to a knock control device for controlling knock occurring in an engine.
2. Description of the Related Art
There is conventionally known a method of detecting a knock phenomenon occurring in an engine by a vibration sensor (hereinafter referred to as a knock sensor) directly mounted on the engine block. It is known that when knock occurs in an engine in operation, a vibration in a specific frequency band arises in a combustion process depending on the bore of the engine and the vibration mode of the knock. This method is to perform, using digital signal processing such as a DFT (discrete Fourier transform), a time-frequency analysis of output from the knock sensor during a period (hereinafter referred to as a knock detection period) that is set in advance and in which vibration is expected to occur due to the knock, thereby calculating a vibration intensity (hereinafter referred to as a knock signal) and detecting the knock based on the calculated knock signal.
The knock is detected based on whether or not the knock signal exceeds a knock determination threshold value; the knock determination threshold value is generally known to be set using a gain and an offset matched in advance with an average value of the knock signal calculated through filtering processing or to be set using the average value of the knock signal and its standard deviation calculated through the filtering processing. Here, the latter will be explained using Equation 1 to Equation 4.
A knock determination threshold value VTH is calculated as follows: Firstly, an average value VBGL of a knock signal VP is calculated through smoothing processing shown by Equation 1, secondly, a variance VVAR of the knock signal VP is calculated through smoothing processing shown by Equation 2 using the average value VBGL and the knock signal VP, and after that, a standard deviation VSGM of the knock signal VP is calculated by calculating the square root of the variance as shown by Equation 3. Then a value obtained by multiplying the standard deviation VSGM by a predetermined coefficient KTH is added to the average value VBGL as shown by Equation 4, so that the knock determination threshold value VTH is calculated.VGBL[n]=KBGL×VP[n−1]−(1−KBGL)×VP[n]  Equation 1where VP is the knock signal; KBGL, a filter coefficient; and n, a stroke number.VVAR[n]=KVAR×VVAR[n−1]−(1−KVAR)×{(VP−VBGL)[n]}2  Equation 2where VVAR is the variance of the knock signal; KVAR, a filter coefficient for calculating the variance; and n, the stroke number.VSGM[n]=(VVAR[n])1/2  Equation 3where VSGM is a standard deviation of the knock signal.VTH=VBGL−KTH×VSGM  Equation 4where VTH is the knock determination threshold value; and KTH, a knock determination threshold value coefficient.
Here, high frequency components are removed from the knock signal VP using a filter coefficient of some 0.95 for each equation, and in addition, when the operation is in a transient state, those filter coefficients are set to smaller than 0.95 for quickly following a knock signal change; meanwhile when the knock is detected, they are set to larger than 0.95 for slowly following the change. Moreover, the knock determination threshold value coefficient KTH is set in advance in such a way that when the knock does not occur, the knock determination threshold value VTH becomes larger than the knock signal VP, whereas when it occurs, the value VTH becomes smaller than the knock signal VP.
Moreover, there is known another nock control device in which when the knock is detected, ignition timing is corrected to shift toward the retard side so as to suppress the knock, whereas when no knock is detected, the ignition timing is returned to the advance side so that torque reduction can be minimized. It is known from general engine characteristics that when the ignition timing is advanced, the knock is easy to occur although output torque from the engine increases, whereas when the ignition timing is retarded, the knock is hard to occur although the output torque from the engine decreases. With this in mind, the nock control device controls the engine in such a way that when the knock is detected, the ignition timing is corrected to shift toward the retard side, whereas when no knock is detected, the ignition timing is returned to the advance side, so that the engine is operated at a knock limit ignition timing that gives maximum torque while suppressing a knock occurrence. However, when the engine is operated under low load, the knock sometimes does not occur even if the ignition timing is advanced to the timing that gives the maximum torque; therefore the above-described knock control is not needed in such an operation range.
In recent years, there have been put into practical use a cylinder injection type engine that, with an injector provided inside a cylinder thereof, injects fuel directly into the cylinder and an engine that is provided with a variable intake/exhaust valve mechanism capable of variably controlling opening/closing timing of the intake and exhaust valves depending on the engine operation state. Particularly in such engines as above, a vibration attributed to fuel injection by the injector (hereinafter referred to as injector noise) and that attributed to the opening/closing of the intake and exhaust valves (hereinafter referred to as valve noise) are easily superimposed on the knock signal.
Furthermore, since the injection timing of the injector and the opening/closing timing of the intake and the exhaust valves are altered in various ways, it is difficult to prevent those kinds of noise from coming in and out of the knock detection period. In a conventional control device, if the noise of those kinds comes in and out of the knock detection period, the knock signal level suddenly changes, and the knock determination threshold value calculated through the filtering processing slowly follows the change; therefore, there has been a problem in that the knock is erroneously detected occurring although no knock occurs.
Meanwhile, a method is proposed in Patent Document 1 in which when the injector noise is superimposed on the proximity of the knock detection period, the knock detection period is altered so that the injector noise is completely included in the period, whereby whether the injector noise is inside the knock detection period or thereoutside is detected separately, the extent of change in the knock signal level due to the injector noise coming in and out is accurately predicted, and the knock determination threshold value is raised depending on the injector noise coming in and out of the period. Moreover, another method is proposed in Patent Document 2, in which the knock detection period is set so as to always include the valve noise, thereby preventing the knock signal from suddenly changing due to the valve noise coming in and out of the period, so that erroneous knock detection can be avoided.    Patent Document 1: Japanese Patent No. 4363171    Patent Document 2: Japanese Laid-Open Patent No. 2008-215142
According to a knock control device of Patent Document 1, although an effect of suppressing erroneous knock detection can be brought about even if the knock signal level changes due to the injector noise coming in and out of the period, the knock signal level itself changes when the injector noise comes in and out of the knock detection period; therefore, processing of raising the knock determination threshold value or processing of prohibiting control of avoiding the knock must be executed. When the knock determination threshold value is raised, the raising quantity needs to be set with some margin taking into consideration fluctuation of the noise level, which has therefore caused a problem in that knock detection is omitted occasionally, and furthermore, when the knock avoiding control is prohibited, there has been another problem in that the knock occurring during the prohibition cannot be detected.
Furthermore, when the knock detection period is expanded, the knock is detected even when noise is superimposed outside a knock occurring period in which the knock does not originally need to be detected; therefore, there has been a problem in that when the vibration level due to the knock is lower than that due to the noise, the knock detection is omitted. Moreover, there has been another problem in that the knock is erroneously detected or the knock detection is omitted for reasons of including in the knock detection period unnecessary noise other than the injector and valve noise.
Moreover, according to a knock control device of Patent Document 2, an effect can be brought about in which erroneous knock detection can be avoided without causing any change in the knock signal level, by avoiding noise itself from coming in and out; however, the knock detection period is expanded to wider than that in Patent Document 1, thereby causing a problem in that the knock detection is omitted when the vibration level due to the knock is constantly lower than that due the noise. Moreover, there has been another problem in that unnecessary noise other than the injector and valve noise is likely to be included in the knock detection period, so that the knock is erroneously detected or knock detection is omitted.
Problems with a conventional knock control device will be explained using FIG. 7A to FIG. 7C and FIG. 8A to FIG. 8C. Here, the explanation will be made on when the conventional device has the same configuration as that of the present invention except for a method of setting a knock window.
FIG. 7A to FIG. 7C each are a diagram showing a spectrum after the digital signal processing in the combustion process, in which (a) to (c) represent noise-superimposed states differing from each other.
Each of (a) to (c) shows a state as follows:
(a): a state in which noise is superimposed on a position distant from a period in which the knock is likely to occur.
(b): a state in which noise is superimposed on the proximity of the period in which the knock is likely to occur.
(c): a state in which noise is superimposed on the period in which the knock is likely to occur.
In each state, a peak-hold value of the spectrum in the knock detection period is calculated as a knock signal.
FIG. 8A to FIG. 8C each are a time chart showing moves of the knock signal average value VBGL and the knock determination threshold value VTH when the noise-superimposed state changes from (a) to (b) to (c).
FIG. 7A and FIG. 8A show the operation of the conventional knock control device. In the conventional knock control device, noise is superimposed outside the knock detection period (closed periods CLS) in (a) and (b) of FIG. 7A; meanwhile in (c) of FIG. 7A, noise is superimposed on the knock detection period OPN (open period OPN). In FIG. 8A, the noise is superimposed on the knock detection period OPN at time T2, and after that, the knock signal VP suddenly increases toward the vibration level due to the noise; however, since the knock determination threshold value VTH that is obtained through smoothing processing of the knock signal VP is calculated with a certain time lag, the knock signal VP exceeds the knock determination threshold value VTH although no knock occurs after time T2, thereby causing erroneous noise detection.
FIG. 7B and FIG. 8B show the operation of the knock control device according to Patent Document 1. In the knock control device according to Patent Document 1, noise is superimposed outside the knock detection period (closed periods CLS) in FIG. 7B (a); meanwhile in FIG. 7B (b), noise existing in the proximity of the knock detection period OPN is detected and the knock detection period OPN is thereby expanded; therefore, the noise is superimposed on the knock detection period OPN, and in FIG. 7B (c), noise is also superimposed on the knock detection period OPN. In FIG. 8B, the noise is superimposed on the knock detection period OPN at time T1 that is earlier timing than time T2 in FIG. 8A, and after that, the knock signal VP suddenly increases toward the vibration level due to the noise. However, level-raising processing is executed on the period between T1 and T1′, and the knock determination threshold value VTH is raised so that a knock determination threshold value VTH′ is calculated, whereby erroneous knock detection can be suppressed. However, the knock determination threshold value is set at an inappropriate level in some cases by the setting of the value VTH′, which causes omission of knock detection. Moreover, when a vibration level due to the knock, VKNK, is the same as the level shown in the figure, for example, the knock cannot be detected after time T1.
FIG. 7C and FIG. 8C show the operation of the knock control device according to Patent Document 2. In the knock control device according to Patent Document 2, the knock detection period OPN is set so as to always include noise thereinside; therefore, the noise is superimposed on the knock detection period OPN in all of (a) to (c) in FIG. 7C. In FIG. 8C, since the knock signal is always held at the vibration level due to the noise, the signal does not suddenly change, thereby avoiding erroneous knock detection. However, when the vibration level due to the knock, VKNK, is the same as the level shown in the figure, for example, the knock cannot be detected at any time.